In a conventional diesel engine, combustion of fuel is generally performed in the form of diffusive combustion.
Tokkai Hei 7-4287 published by the Japanese Patent Office in 1995 has proposed applying low temperature premixed combustion to a direct injection diesel engine in order to reduce nitrogen oxides (NOx) and particulate matter (PM). According to this prior art technique, the oxygen concentration of cylinder intake air is reduced and the combustion temperature of the air-fuel mixture is reduced by an exhaust gas recirculation (EGR) system. The ignition delay of the injected fuel is largely lengthened, and mixing of fuel and cylinder intake air before ignition is promoted.
The fall of combustion temperature causes a reduction of NOx, and promotion of fuel and cylinder intake air before ignition causes a reduction of PM.
In order to realize low temperature premixed combustion, the combustion temperature and ignition delay period must be controlled within fixed limits. In the heavy load region or high rotation speed region of an engine, it is difficult to satisfy these conditions due to temperature rise of the recirculated exhaust gas. Thus, in diesel engines using this prior art technique, diffusive combustion is performed in the high load region or high rotation speed region.